1. Field of the Invention
The present invention relates generally to methods and systems for investigating subterranean formations. More particularly, this invention is directed to methods and systems for characterizing hydraulic fracture networks in a subterranean formation.
2. State of the Art
In order to improve the recovery of hydrocarbons from oil and gas wells, the subterranean formations surrounding such wells can be hydraulically fractured. Hydraulic fracturing is used to create cracks in subsurface formations to allow oil or gas to move toward the well. A formation is fractured by introducing a specially engineered fluid (referred to as “hydraulic fluid” herein) at high pressure and high flow rates into the formation through one or more wellbore. Hydraulic fractures typically extend away from the wellbore hundreds of feet in two opposing directions according to the natural stresses within the formation. Under certain circumstances they instead form a complex fracture network.
The hydraulic fluids are typically loaded with proppants, which are usually particles of hard material such as sand. The proppant collects inside the fracture to permanently “prop” open the new cracks or pores in the formation. The proppant creates a plane of high-permeability sand through which production fluids can flow to the wellbore. The hydraulic fluids are preferably of high viscosity, and therefore capable of carrying effective volumes of proppant material.
Typically, the hydraulic fluid is realized by a viscous fluid, frequently referred to as “pad” that is injected into the treatment well at a rate and pressure sufficient to initiate and propagate a fracture in hydrocarbon formation. Injection of the “pad” is continued until a fracture of sufficient geometry is obtained to permit placement of the proppant particles. After the “pad,” the hydraulic fluid typically consists of a fracturing fluid and proppant material. The fracturing fluid may be a gel, an oil base, water base, brine, acid, emulsion, foam or any other similar fluid. The fracturing fluid can contain several additives, viscosity builders, drag reducers, fluid-loss additives, corrosion inhibitors and the like. In order to keep the proppant suspended in the fracturing fluid until such time as all intervals of the formation have been fractured as desired, the proppant should have a density close to the density of the fracturing fluid utilized. Proppants are typically comprised of any of the various commercially available fused materials such as silica or oxides. These fused materials can comprise any of the various commercially available glasses or high-strength ceramic products. Following the placement of the proppant, the well is shut-in for a time sufficient to permit the pressure to bleed off into the formation. This causes the fracture to close and exert a closure stress on the propping agent particles. The shut-in period may vary from a few minutes to several days.
Current hydraulic fracture monitoring methods and systems map where the fractures occur and the extent of the fractures. The methods and systems of microseismic monitoring process seismic event locations by mapping seismic arrival times and polarization information into three-dimensional space through the use of modeled travel times and/or ray paths. These methods and systems can be used to infer hydraulic fracture propagation over time.
Conventional hydraulic fracture models typically assume a bi-wing type induced fracture. They are short in representing the complex nature of induced fractures in some unconventional reservoirs with preexisting natural fractures such as the Barnett Shale and many other formations. Several recently published models map the complex geometry of discrete hydraulic fractures based on monitoring microseismic event distribution. They are typically not constrained by accounting for either the amount of pumped fluid or mechanical interactions both between fractures and injected fluid and among the fractures. Those few better constrained models have greatly improved our fundamental understanding of involved mechanisms. However, they are inevitably complex in mathematical description and often require substantial computer processing resources and time in order to provide accurate simulations of hydraulic fracture propagation.